1. data is not gonna lose
2. mount the database
3. issue "alter database datafile end backup"
4. then take the datafile offline and restore and/or recover it
5. Then bring the datafile online.
When a tablespace is in hot backup mode, and the database crashes, start up the database in mount mode. Query from V$BACKUP to find out which data files (also tabelspace) are in backup mode, then issue ALTER TABLESPACE END BACKUP; then ALTER DATABASE OPEN;
You do not need to recover the database/datafile.